1. Field of the Invention
The present invention relates to measurement equipment, and more particularly to compensating pendulum-type linear accelerometers with flexible suspension of the sensing element.
2. Description of the Related Art
An example of a conventional accelerometer is illustrated in Japanese Patent No. 188924/1981. This accelerometer includes an inertial mass (in this case, a pendulum) that is placed in a sealed housing. Inside the sealed housing there is typically a vacuum or an inert gas, such as helium. The inertial mass is generally in the shape of a movable rod, whose lower portion is attached, using flexible suspension, to a plate. The plate is then affixed, using screws, to a base surface of one of the frames. The rod can rotate, using the flexible suspension, around the plate in the direction of the measurement axis of the accelerometer. The rod of the pendulum has two cylindrically shaped coils attached to it.
The axial magnetic systems of the accelerometer include magnetic conductors, in this case first and second frames, which are made of magnetically soft material. The magnetic axial systems also include permanent magnets and field concentrators, such that the movable coils are located in the gaps formed between the surfaces of the magnetic conductors and the field concentrators. The above elements together with the movable coils comprise the momentum sensor of the accelerometer.
Both frames are rigidly connected to each other, and each of the frames includes stoppers for limiting the range of motion of the pendulum. This range of motion can be regulated by moving the stoppers in the threaded openings in the frames.
Inside the housing there is an angle sensor that measures the movement of the pendulum. The angle sensor consists of a single light source and two light detectors. The pendulum is located between the frames, such that the light source is located on one side of the pendulum, and the light detectors are located on the other side of the pendulum. Thus, the pendulum acts as a shading element that blocks the light from the light source to the light detectors. When there is no measured acceleration along the measurement axis, the pendulum is in a neutral position, and some of the light from the light source is shielded by the pendulum, while the remainder of the light is evenly distributed between the two light detectors. Thus, the current produced by the light detectors (if, for example, photodiodes are used) is equal. Note that the surfaces of the photodiodes (light detectors) need not be entirely illuminated, but so long as the illumination of each detector is the same, the output currents are the same, indicating that the measured acceleration is zero.
When the sensor experiences acceleration along the measurement axis, the pendulum is displaced from its neutral position due to inertia force. As a result, the light distribution between the two detectors changes, and therefore the relationship between the current from the two detectors also changes. The difference between the two currents is related to the acceleration, and using conventional electronics, can be converted to an acceleration value. The current can also be used in a feedback circuit; it passes through the movable coils interacts with the magnetic field of the permanent magnets and returns the pendulum to its neutral position. The magnitude and polarity of the feedback current therefore permit a measurement of the acceleration.
The conventional accelerometer described above has a number of disadvantages. One of the disadvantages is that due to the relatively large dimensions of the rod of the pendulum, the light detectors need to be placed relatively far apart, which tends to increase noise and reduces the sensitivity of the measurement. Another disadvantage is that the neutral position of the pendulum typically does not precisely correspond to a zero output signal of the accelerometer. Yet another disadvantage is that mechanical tuning of the accelerometer tends to be difficult, particularly with regards to the zero bias signal.
Another example of a conventional accelerometer is described in U.S. Pat. No. 4,649,748. The accelerometer described in this patent shares some commonalities with the earlier-described accelerometer. However, the accelerometer in U.S. Pat. No. 4,649,748 has the following differences compared to the device of Japanese Patent No. 188924/1981: first, the first frame—the magnetic conductor of the magnetic system—is used as a mounting base of the accelerometer, which permits simplifying the manufacturing of the accelerometer, and also reduces its mass.
Additionally, the free end of the pendulum rod, which is located between the light source and the light detectors, is formed as a thin plate which permits the light source and the light detectors to be closer to each other. This increases the sensitivity of the angle sensor, and reduces the noise in its output. One of the options is for the thin plate to also have the shape of a rod, another option is to have the plate shaped as a thin rod with a slit along the axis of the rod, such that part of the light passes through the slit.
The various elements of the overall construction where the light source and light detectors are housed are pressed against the frames using a flat spring, which increases the precision of the mechanical tuning of the zero bias signal.
The device described in U.S. Pat. No. 4,649,748 has a number of disadvantages. For example, one of the disadvantages is due to the use of two magnetic systems. In case of a need to increase range of measured acceleration, this leads to a relatively values for currents that might flow through the coils, which in turn leads to a large amount of heat being dissipated, which in turn leads to an error source in the measurement, relating to the waste heat. Additionally, the use of two different magnetic systems increases the manufacturing cost and complexity of the device. These disadvantages are also present in the device described in Japanese Patent No. 188924/1981, discussed above.
Furthermore, the mechanical tuning of the zero bias signal is relatively coarse. Furthermore, a relatively small range of displacement of the light source and detectors relative to the housing makes it difficult to tune the angle sensor when there is a relatively large angle between the axis of the pendulum rod in the neutral position, and the base mounting surface of the accelerometer.